Why the total mass never changes in a reaction, why a balance reading can still go up or down, and how to add up a relative formula mass.
What you will learn
- Explain why mass seems to change when a gas escapes or is taken in
- Calculate Mr for formulae with brackets and hydrates
- Use total Mr values to show mass is conserved in an equation
Atoms are only rearranged
In a chemical reaction the atoms swap partners.
None are made and none are destroyed, so the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass, and it is why a symbol equation must balance.
Yet the reading on a balance can change. That happens when the container is open and a gas is involved:
- Mass goes down when a gas is made and escapes. Heating copper(II) carbonate gives off carbon dioxide, so the solid left is lighter.
- Mass goes up when a gas from the air joins on. Magnesium burning in air takes in oxygen, so the magnesium oxide is heavier than the magnesium was.
- Marble chips and acid in an open flask: reading falls from 152.40 g to 151.96 g
- mass of carbon dioxide that escaped = 152.40 − 151.96 = 0.44 g
- In a sealed flask the reading would not change at all
The mass is not lost. It has moved somewhere the balance cannot weigh it.
The rest of this lesson and its 8 practice questions are locked.
In this lesson
- Atoms are only rearranged
- Relative formula mass, Mr
- Mass balances in an equation
- Try it: Check your Mr with the calculator
- Try it: Your first Mr
8 practice questions, marked as you go; about 20 minutes in all.
Key terms
Mass goes down, Mass goes up, No unit
In this unit: Quantitative chemistry and the mole
Conservation of mass, relative formula mass, percentages and concentrations, the mole, reacting masses, empirical formulae, yield, atom economy, titrations and gas volumes.